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IGL + Concepts = Cleaner function declarations
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| #include <Eigen/Core> | |
| #include <concepts> | |
| #include <iostream> | |
| // This example code shows how concepts could be used to lighten the syntax used | |
| // in declaring igl style function input/outputs. In particular, inputs are | |
| // typically derived from MatrixBase (which can be accessed directly) and | |
| // outputs are derived from PlainObjectBase (which has memory to store values). | |
| // | |
| // Below there are three examples: | |
| // assign, assign_auto, assign_igl_sugar, which show 3 different syntaxes for | |
| // using constraints to check the validy of igl-style function input/outputs. | |
| namespace concepts { | |
| namespace detail { | |
| // Eigen uses CRTP, so A is declared in the form | |
| // > class A: public Base<A>, so A is derived from Base<A>. | |
| template <typename T> | |
| concept MatrixBaseDerived = std::derived_from<T, typename Eigen::MatrixBase<T>>; | |
| template <typename T> | |
| concept PlainObjectBaseDerived = | |
| std::derived_from<T, typename Eigen::PlainObjectBase<T>>; | |
| // We sanitize inputs to the above constraints for When the input passed is not | |
| // the most derived type, which can happen when an igl-style function calls | |
| // another igl-style function. | |
| template <typename T> | |
| using derived_type = std::decay_t<decltype(std::declval<T>().derived())>; | |
| } // namespace detail | |
| // Here's the most basic sort of constraint | |
| template <typename T> | |
| concept MatrixBaseDerived = detail::MatrixBaseDerived<detail::derived_type<T>>; | |
| template <typename T> | |
| concept PlainObjectBaseDerived = | |
| detail::PlainObjectBaseDerived<detail::derived_type<T>>; | |
| } // namespace concepts | |
| // We can now declare the types in the template declaration rather than in the | |
| // function declaration, which hopefully cleans the specification a bit | |
| template <concepts::MatrixBaseDerived A, concepts::PlainObjectBaseDerived B> | |
| void assign(const A& a, B& b) { | |
| b = a; | |
| } | |
| // We can also hide the template declaration completely and move constraints | |
| // into the function body | |
| void assign_auto(const auto& a, auto& b) { | |
| static_assert(concepts::MatrixBaseDerived<decltype(a)>); | |
| static_assert(concepts::PlainObjectBaseDerived<decltype(b)>); | |
| b = a; | |
| } | |
| // We can alias these to make the syntactic sugar a bit nicer | |
| namespace concepts { | |
| template <typename T> | |
| concept IglInputType = MatrixBaseDerived<T>; | |
| template <typename T> | |
| concept IglOutputType = PlainObjectBaseDerived<T>; | |
| } // namespace concepts | |
| // With these convenience functions the intent of IGL style input/output are | |
| // very clear | |
| template <concepts::IglInputType A, concepts::IglOutputType B> | |
| void assign_igl_sugar(const A& a, B& b) { | |
| b = a; | |
| } | |
| int main(int argc, char* argv[]) { | |
| auto a = Eigen::Matrix3d::Identity(); | |
| Eigen::MatrixXd b; | |
| // basic example | |
| assign(a, b); // passes | |
| std::cout << b << std::endl; // outputs 1,0,0;0,1,0;0,0,1 | |
| assign_auto(a, b); // passes | |
| std::cout << b << std::endl; // outputs 1,0,0;0,1,0;0,0,1 | |
| // we can also use a bit of syntactic sugar to further simplify things | |
| assign_igl_sugar(a, b); // passes | |
| std::cout << b << std::endl; // outputs 1,0,0;0,1,0;0,0,1 | |
| // a slightly more complex example | |
| assign((a.array() + 1).matrix(), b); // passes | |
| std::cout << b << std::endl; // outputs 2,1,1;1,2,1;1,1,2 | |
| // This doesn't work because the RHS is a read-only expression | |
| // assign(a, b + a); // fails | |
| } |
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